Perihelion Night on the wooded moon Verd. A time of strange sightings, ghosts, and celebration before the morn, when Lord Esnic marries the beautiful Lady Ria.

However Ria has other ideas, and flees through the gravity wells which dot the moon to meet with her true love Tonio.

When the Doctor and Jo arrive on Verd, drawn down by the fluctuating gravity, they find themselves involved in the unpredictable events of Perihelion. But what of the mysterious and terrifying Nightdreamers? And of the Nightdreamer King?